Mitochondria are found in nearly all eukaryotic cells and vary in number and location depending on the cell type. Mitochondria contain their own DNA (mtDNA) and their own machinery for synthesizing RNA and proteins. The mtDNA contains only 37 genes, thus most of the gene products in the mammalian body are encoded by nuclear DNA.
Mitochondria perform numerous essential tasks in the eukaryotic cell such as pyruvate oxidation, the Krebs cycle and metabolism of amino acids, fatty acids and steroids. The primary function of mitochondria is the generation of energy as adenosine triphosphate (ATP) by means of the electron-transport chain and the oxidative-phosphorylation system (the “respiratory chain”). Additional processes in which mitochondria are involved include heat production, storage of calcium ions, calcium signaling, programmed cell death (apoptosis) and cellular proliferation. It has been disclosed that mitochondria have a role in cell regulatory and signaling events (i.e. regulation of Ca2+ fluxes, oxidative stress and energy-related signaling among others). Therefore, there are many diseases and disorders known in the art associated with malfunction or dysfunction of mitochondria which require efficient treatment.
U.S. Pat. No. 7,279,326 discloses a composition for delivering a wild-type mitochondrial DNA genome to a mammalian cell, the composition comprising: a wild-type mammalian mitochondrial DNA genome molecule; a mitochondrial leader sequence peptide attached to the mitochondrial DNA genome molecule; and a carrier vehicle comprising two delocalized cationic centers separated by a hydrocarbon chain.
Attempts to induce transfer of mitochondria into host cells or tissues have been reported. Most methods require active transfer of the mitochondria by injection (e.g. McCully et al. Am J Physiol Heart Circ Physiol. 2009, 296(1):H94-H105; Van Blerkom et al. Hum Reprod. 1998, 13(10):2857-68; Pinkert et al. Transgenic Res. 1997, 6(6):379-83; King et al. Cell 1988, 25; 52(6):811-9; Abramova et al. Biokhimiia. 1983 August; 48(8):1279-86 and Abramova et al. Ontogenez. 1979; 10(4):401-5). Transfer of mitochondria engulfed within a vehicle, such as a liposome, is also known (e.g. Shi et al. Ethnicity and Disease, 2008; 18(S1):43).
It has been shown that mitochondrial transfer may occur spontaneously between cells in vitro although it was only established that mtDNA was transferred rather than intact whole functional mitochondria (e.g. Plotnikov et al. Exp Cell Res. 2010, 316(15):2447-55; Spees et al. Proc Natl Acad Sci, 2006; 103(5):1283-8). Mitochondrial transfer in-vitro by endocytosis or internalization has been demonstrated as well (Clark et al., Nature, 1982; 295:605-607; Katrangi et al., Rejuvenation Research, 2007; 10(4):561-570).
U.S. 2010/0278790 discloses rescue of ischemic cardiac cells by transfer of mesenchymal stem cells. Without limiting the invention to a particular mechanism, since loss of functional mitochondria is an early consequence of ischemia, MSCs may in part rescue ischemic injury to myocardium or other tissues by transfer of mitochondria or mtDNA.
U.S. 2011/0008310 discloses methods, kits and compositions for mitochondrial replacement in the treatment of disorders arising from mitochondrial dysfunction. This disclosure provides isolated and substantially pure mitochondria for implantation into cells. U.S. 2012/0058091 discloses, inter-alia, an oocyte, an embryonic cell and a modified stem-cell comprising heterologous mitochondria. The mitochondria are delivered in some instances by microinjection in vitro.
Attempts have also been made to use mitochondria that underwent cryopreservation. It has been shown that mitochondria that have been frozen in a buffer comprising trehalose retain a number of biological functions and preserve outer membrane integrity, while showing significantly decreased rates of both phosphorylating and maximally uncoupled respiration (Yamaguchi et al., Cell Death and Differentiation, 2007, (14):616-624). U.S. 2011/0105359 provides cryopreserved compositions of cells in the form of self-sustaining bodies, as well as cellular and subcellular fractions. On the other end, an attempt to inject isolated mitochondria during early reperfusion for cardioprotection showed that cardioprotection requires freshly isolated mitochondria, as frozen mitochondria failed to provide cardioprotection and displayed a significantly decreased oxygen consumption compared with freshly isolated mitochondria (McCully et al., Am. J. Physiol. Heart Circ. Physiol., 2009, (296): H94-H105). Thus, the oxygen consumption rate and functionality of frozen mitochondria is still under debate.
However, there remains an unmet need for an effective and reproducible therapeutic treatment of diseases and disorders associated with nonfunctional or dysfunctional mitochondria.